1. Field of the Invention
The present invention relates to an improvement in a laser beam direction correcting optical system for a surveying instrument which emits a laser beam in a specific direction.
2. Description of the Related Art
Some of the surveying instruments have a rotating laser emitting device which rotates laser light within a horizontal plane to generate a reference plane. FIG. 9 shows an example of the rotating laser emitting device. This rotating laser emitting device is equipped with a visible semiconductor laser 1 as a laser light source, a collimator lens, an incident angle correcting means 3, and a laser rotating-scanning means 4.
The visible semiconductor laser 1 is driven by a pulse drive circuit 5 consisting of an oscillator 5a and an LD drive circuit 5b. The collimator lens 2 converts the laser light emitted from the visible semiconductor laser 1 to a collimated light beam P1. The collimated light beam P1 is incident upon the incident angle correcting means 3.
The laser rotating-scanning means 4 is equipped with a vertical-direction orienting means 6 and a pentagonal prism 7 which reflects the laser light incident upon the orienting means 6 in a horizontal direction. The pentagonal prism 7 is mounted on a rotary mounting table 8 so that it can rotate. With rotation of this pentagonal prism 7, the laser light is rotated within a horizontal plane and emitted to an irradiated object.
The incident angle correcting means 3 is equipped with an in-X plane inclination correcting means and an in-Y plane inclination correcting means perpendicular to the in-X plane inclination correcting means. The in-X plane inclination correcting means is constituted by an oil bath 10, enclosed glasses 11 and 12, and oil 13 enclosed within the oil bath 10. The in-Y plane inclination correcting means is constituted by a pair of prisms 14 and 15. The oil 13 is horizontally maintained regardless of an inclination of the instrument main body. The direction of reflection of the collimated light beam P1 within an X plan is corrected according to an inclination of the instrument main body by the back surface 13a of the oil 13, and the corrected light beam P1 is guided to the in-Y plane inclination correcting means. The inclination of the guided light beam P1 within a Y plane is corrected by the in-Y plane inclination correcting means, and the corrected light beam P1 is guided to the vertical-direction orienting means 6. The incident angle correcting means 3 is interposed between the visible semiconductor laser 1 and the vertical-direction orienting means 6. The incident angle correcting means 3 fulfills a role of correcting an incident angle of the collimated light beam P1 incident upon the vertical-direction orienting means 6 in accordance with an inclination of the instrument main body.
The vertical-direction orienting means 6 consists of a reflecting mirror 16 and an exit angle correcting means 17. The exit angle correcting means 17 consists of a first objective lens 18 and a second objective lens 19. This exit angle correcting means 17 fulfills a role of correcting an exit angle of the laser light beam emerging from the second objective lens 19 in accordance with an inclination of the instrument main body so that the laser light beam is oriented in a specific direction (e.g., a vertical direction).
Incidentally, in surveying instruments, a distance to an irradiated object is not constant, and in order to make emission efficiency variable in accordance with each distance, there have been demands that a function of focusing laser light on an irradiated object be added to the exit angle correcting means 17. More specifically, there have been demands that laser light be emitted to an irradiated object not as a collimated light beam but as an image forming light beam.
Particularly, in the case of visible laser light, if laser light is emitted as a collimated light beam, the laser light will become more difficult to see as distance increases. For this reason, it is desirable that the image of laser light be formed in an irradiated object.
In the conventional rotating laser light emitting device, however, laser light is incident upon the exit angle correcting means 17 after it has been corrected by the incident angle correcting means 3. For this reason, if the first objective lens 18 of the exit angle correcting means 17 is moved in the optical axis O of the correcting means 17 so that laser light is focused on an irradiated object, then the laser light emerging from the second objective lens 19 will be shifted from a specific direction. Therefore, the problem of degrading correction accuracy in the exit angle of an outgoing light beam arises for the following reasons.
As shown in FIG. 10(a), the incident angle of a collimated light beam incident upon the first objective lens 18 relative to the optical axis O of the exit angle correcting means 17 is taken to be .theta..sub.in. Also, the exit angle of the collimated light beam P1 emerging from the second objective lens 19 relative to the optical axis O of the exit angle correcting means 17 is taken to be .theta..sub.out. The focal length of the first objective lens 18 is taken to be fa, and the focal length of the second objective lens 19 is taken to be fb. Assuming the focal point of the first objective lens 18 and the focal point of the second objective lens 19 are consistent with each other, the image of the collimated light beam incident upon the first objective lens 18 will be formed temporarily at the focal position fa and have an image height Y. Then, the incident light is emitted again from the second objective lens 19 as a collimated light beam. At this time, between the incident angle .theta..sub.in of the incident light beam and the exit angle .theta..sub.out of the outgoing light beam, the following relation is established: EQU .theta..sub.out =r.multidot..theta..sub.in
where r is angular magnification (fa/fb).
Next, as shown by a broken line in FIG. 10(b), by moving the second objective 19 by .DELTA.X in a direction of arrow along the optical axis O, the image of the laser light is formed in an irradiated object. In this case, in accordance with a distance from the second objective lens 19 to the irradiated object, an error .DELTA..theta..sub.out occurs in the exit angle .theta..sub.out (S) of an outgoing light beam when the outgoing light beam is emitted to infinity, so that correction accuracy in the exit angle of the outgoing light beam is degraded. Reference character S denotes a distance from the second objective lens 19 to an irradiated object in the optical axis direction (i.e., a distance to an image-formed position).